Contact lenses are traditionally manufactured by several means including "lathing", "spin casting" and "cast molding". Each of the above methods possesses advantages in terms of the cost of production of lenses or the variety of lens designs and materials which may be produced. Cast molding offers significant advantages in respect of relatively low cost of capital plant employed in the production process as well as low unit cost of production while being utilisable over a wide range of polymeric materials.
Present methods of cast molding described in U.S. Pat. No. 4,208,364 (Shepherd) and U.S. Pat. No. 4,284,399 (assigned to American Optical Corporation) suffer in practice from relatively low production yields due to defects in or originating from the edge of the cast lens, and in the case of the method described in U.S. Pat. No. 4,209,289 (assigned to American Optical Corporation), from numerous instances of poor optical quality.
U.S. Pat. No. 4,208,364 (Shepherd) teaches the casting of a lens between two disposable plastic mold members one of which is provided with a deformable lip which facilitates the relative movement of the mold members towards each other in order to compensate for the shrinkage of the lens polymer which occurs during polymerization. Said movement maintains contact between the optical surfaces of the mold members and the lens polymer thus ensuring good optical quality of the lens. However, said deformable lip against which the edge of the lens is formed, being of a disadvantageous cross-sectional form and minute dimensions, is difficult to produce to the required degree of precision by the specified injection molding process.
The resulting lens edges frequently exhibit imperfections which may become more pronounced during the process of removing the formed lens from the assembled mold members. Such imperfections existing in the edge of the lens often cause the lens to be judged as unfit for use. In addition, such imperfections often form sites for the initiation of cracks which may propagate into the lens providing further cause for rejection of the lens on inspection.
A further limitation of the method taught in the Shepherd patent results from variable deformation of the described flexible lip which in turn results in variation of the edge thickness around the lens and, at times, in unacceptable variation in the lens centre thickness.
As a result of the above deficiencies, the production yield of lenses manufactured by the Shepherd method, being the number of lenses produced from a given number of cast moldings, is generally of the order of 50% or less.
The method described in U.S. Pat. No. 4,284,399 (assigned to American Optical Corporation) does not provide a means for the mold members to move towards each other during polymerization other than by deformation of the surfaces of the mold members, which appears to be assisted by the loading of the assembled members with a weight of "two to three pounds". The deformation of the surfaces of the mold members can be expected to result in loss of optical quality in the molded lens.
The method can further suffer from imperfections in the lens edge which is formed against the junction line between the two mold members. Misalignment of the mold members on assembly of the members prior to polymerisation of the lens forming monomer may occur due to variation in the actual size of the mold members produced from given tooling at different times. Any such misalignment will result in a deformation of the lens edge.
A further limitation of the method described in the American Optical patent lies in the fact that the configuration of the portion of the female mold member at the point where the edge is formed against such member, being of disadvantageous cross-sectional form and minute dimensions, does not lend itself to production by the injection molding process specified in the patent. The lens edge form shown in the patent could not therefore be effectively molded without deformation resulting from imperfections in the said portion of the female mold member.